I think there are benefits to alliances by limiting their size. Realistically in a 100-member alliance about 20-30 people are most active in chat and other alliance activities. Managing a 100-member alliance is already a challenge -- managing more than that would go from "occasionally a drag" to "total nightmare." The larger an organization is, the easier it is for some of the folks to fall through the cracks -- this is already a problem with 100 members and would be worse with 200.
I've led alliances with 200 players in other games, and I think a lower limit makes sense.
In addition, even alliances that share the same ideas about things like inter-alliance politics can have very different characteristics. The Crow Federation alliances share fairly similar ideals but attract players with different goals and interests, just as one example.
That doesn't mean I'm absolutely opposed to increasing the membership limit on alliances, just that I see a number of valid reasons not to and no compelling argument for doing so -- especially after player-run chats are introduced.